I have a piece of code where I fill variables with data
and would like to add the first column of each row into the variable:
So each variable in a row should contain: %VAR_ + 'value of column[0]' + headerRow[i] + %
Now it looks like this which makes variables only unique within a row.

result = []
for line in lines:
i = 0
for cell in row:
tmp = ('%VAR_' + headerRow[i] + '%')
line = line.replace(tmp, cell) # string.replace(old, new)
i = i + 1
result.append(line)
return result

Note that you will replace tmp with cell no matter when it is found, i.e the for loop will always replace tmp with cell 1 on the first pass if it is found, no matter where in the line it occurs. Without more info about what input and output should be, and what row contains, there is nothing more that can be said.